In evaporation methods a substrate is placed inside a vacuum chamber. A source material which is to be deposited onto the substrate is also arranged inside the vacuum chamber. The source material is heated to the point where it starts to evaporate. The vacuum allows atoms or molecules to evaporate freely in the chamber and the atoms and molecules subsequently condense on substrate surfaces. This principle is the same for all evaporation methods, only the method used to evaporate the source material differs. There are two popular evaporation methods, which are e-beam evaporation and resistive evaporation. In resistive evaporation, the source material is heated electrically with a high current to make the material evaporate. Molecular beam epitaxy is an evaporation method, which is characterized by a slow deposition rate, which allows a film to grow on the substrate epitaxially i.e. through ordered crystalline growth.
Molecular beam epitaxy (MBE) is a technique for epitaxial growth via the interaction of one or several molecular or atomic beams that occurs on a surface of a heated crystalline substrate. In an MBE vacuum chamber the material to be deposited is heated in a crucible, evaporated and the evaporated molecular beam is directed to a heated crystalline substrate. The evaporated material is in atomic or molecular form depending on the material. The evaporated materials then condense on the substrate, where they may react with each other. The deposition or growth rate is controlled by the temperature of the crucible and mechanical shutters are used to switch the deposition on and off. Typically a plurality of sources are mounted in the deposition chamber in order that several different materials can be evaporated sequentially or simultaneously. MBE is widely used in the semiconductor research and in the semiconductor device fabrication industry. In a typical research MBE system the substrate is facing down, inclined from horizontal with growth surface facing down or the substrate is vertically mounted as shown in the Prior Art FIGS. 1 and 2.
In Prior Art production type MBE devices effusion cells are directed at the substrate at an angle of approximately 45° from the normal of the substrate surface. Conical crucibles are used to provide good uniformity over large area substrates. The patent U.S. Pat. No. 5,827,371 discloses a one-piece monolithic crucible for an MBE effusion source. The maximum temperature of such a crucible made of pyrolytic boron nitride (PBN) is 1400° C. Above this temperature PBN starts to decompose to Boron and Nitrogen. Many applications need higher than 1400° C. temperatures. For example SrTiO2 (STO) layer growth requires Titanium temperatures of 1500-1700° C. STO based technology has major applications for high-k dielectrics in the silicon industry, k referring to a dielectric constant of a material.
Certain high vapor pressure materials such as arsenic, phosphorus, antimony can also be evaporated using a thermal cracker source which cracks the evaporated source materials to dimers or monomers. These cracker sources have heated crucibles to evaporate the source material, a cracker stage to crack the molecules to dimers or monomers and a control valve between the crucible and the cracker to control the effusion rate from the cracker. A description of such cracker source for phosphorus is contained in US20080019897A1.
A problem associated with existing production MBE vacuum chambers is that the substrate growth surface is facing down which requires the substrates to be loaded and unloaded into the vacuum chamber using carrier rings. The downward facing growth is not compatible with existing semiconductor process equipment which typically have the growth surface facing up and which do not use substrate carriers.
In Prior Art MBE effusion cells the flux rate from the source depends on the height of the material level in the crucible. Prior Art MBE effusion cells have a limited crucible capacity when using liquid source materials. The material flux out of the crucible consists of two parts: atoms or molecules emitted from the melt level directly to the substrate without reflections from the crucible walls and atoms or molecules going through one or more reflections from crucible walls before reaching the substrate. With prior art crucibles the depletion of the source material causes the surface area of the melt to diminish, which in turn may lead to a diminishing material flux to the substrate.
In MBE vacuum chambers, large amounts of deposits are accumulated on the chamber walls and other structures inside the deposition chamber over time. These deposits are typically loosely attached and fall down easily. Any deposit falling back in to the crucible will contaminate the source material and result in impurities and defects in the deposited thin film.
A silicon substrate may develop a dislocation called “slip” while subjected to high-temperature processing. If such a slip extends into an integrated circuit, the circuit will suffer failures like increased leakage and dielectric breakdown. Such a defect is more likely to occur if the silicon substrate is held in place from the substrate edges with a carrier ring used in a typical MBE apparatus. Damage to silicon wafers caused by gravity when the wafer is held from the edges only is discussed, for example in U.S. Pat. No. 7,022,192. In Prior Art MBE systems the silicon wafer cannot be supported at any other point because this would damage the growth surface. In the present invention the wafer can be held from the back side so there is no damage to the front side growth surface.